Rostro-caudal specificity of corticospinal tract projections in mice

ABSTRACT Rostro-caudal specificity of corticospinal tract (CST) projections from different areas of the cortex was assessed by retrograde labeling with fluorogold and retrograde transfection following retro-AAV/Cre injection into the spinal cord of tdT-reporter mice. Injections at C5 led to retrograde labeling of neurons throughout forelimb area of the sensorimotor cortex, the rostral forebrain area (RFA), and a region in the lateral cortex near the barrel field. Injections at L2 led to retrograde labeling of neurons in the posterior sensorimotor cortex (hindlimb area) but not the RFA or lateral cortex. With BDA injections into the main sensorimotor cortex (forelimb region), labeled axons terminated selectively at cervical levels. With BDA injections into caudal sensorimotor cortex (hindlimb region), labeled axons passed through cervical levels without sending collaterals into the gray matter and then elaborated terminal arbors at thoracic-sacral levels. With BDA injections into the RFA and lateral cortex near the barrel field, labeled axons terminated at high cervical levels. Axons from medial sensorimotor cortex terminated primarily in intermediate laminae; Axons from lateral sensorimotor cortex terminated primarily in deep layers of the dorsal horn. One of the descending pathways seen in rats (the ventral CST) was not observed in most mice. SIGNIFICANCE Mice are used extensively for studies of regeneration following spinal cord injury because of the ability to create genetic modifications to explore ways to enhance repair and enable axon regeneration. A particular focus has been the corticospinal tract (CST) because of its importance for voluntary motor function. Here, we document features of the rostro-caudal specificity of CST